Thursday 24 November 2011 21.36 EST
First published on Thursday 24 November 2011 21.36 EST

Twenty-six bodies were discovered bound and gagged on Thursday in vehicles abandoned in the heart of Guadalajara, Mexico's second-largest city and the site of the recent Pan American Games, officials said.

Best known as the home of mariachi music and tequila, this picturesque city has also been the historic base for methamphetamine trafficking by the powerful Sinaloa cartel. The cartel's tight grip on the city was shattered by the death of its regional commander, Ignacio "Nacho" Coronel, in a shootout with federal police in July 2010.

Guadalajara's murder rate then soared as factions of the cartel known as the New Generation and the Resistance battled to control Coronel's territory and assets. Street battles have left hundreds dead in the city and surrounding areas.

Security officials have said they feared that the chaos could provide an opening for the Zetas drug cartel, which has been using paramilitary-style tactics and headline-grabbing atrocities in a national push to seize territory from older organized crime groups.

But killing slowed to a trickle during the Pan American Games in late October, which brought a massive influx of police and soldiers.

Now, the violence appears to have surged back, in the form of the mass killing and public dumping of bodies that has marred other cities such as Veracruz.

The state prosecutor's office said the dead men in Guadalajara were found in two vans and a pickup truck near the Millennium Arches, one of the most recognisable landmarks in the city in western Mexico.

The arches stand less than a mile from the Expo Guadalajara events centre, the site of the Pan Am Games and the Guadalajara international book fair, which opens on Saturday and describes itself as the world's most important Spanish-language book fair. The fair's website said it was expecting more than 600,000 visitors from around the world. On Wednesday, 17 bodies were found burned in two pickup trucks in a strikingly similar attack in Sinaloa, the home state of the eponymous cartel. Twelve of the bodies were in the back of one truck, some of them handcuffed and wearing bulletproof vests.

Luis Carlos Najera, public security secretary for the state of Jalisco, where Guadalajara is located, said on Thursday morning that a message had been found in one of the vehicles containing the most recent bodies, but did not offer more details. Mexican drug cartels frequently leave threatening messages with the bodies of their victims as a way of sowing fear and taking credit for their actions.

Responding to a reporter's question, Najera told the Televisa television network that he believed the recent calm in Guadalajara was the result of an increase in security and not because drug cartels had struck a truce with each other during the games.

He declined to comment on the possible motives for the slayings, saying only that investigators had "various hypotheses."

The Zetas have taken over neighboring Zacatecas state in their push west, and are said to be eyeing Guadalajara both for the meth trade and for the potential of extortion.

Analysts have said there is some indication that factions such as the Resistance will join the Zetas, which would produce a coalition threatening Sinaloa's methamphetamine operations.